1. Field of the Invention
The present invention relates to a method for electrostatically coating a workpiece with paint, comprising the following steps: mixing with air a paint in the form of a powder consisting of a plurality of particles dispersed in air; electrostatically charging the individual particles forming the powder paint by submitting the paint to an electric ionization field; projecting the electrostatically-charged powdered paint against a workpiece at the same time as said air is ejected through a spraying nozzle.
The invention also relates to an apparatus for electrostatically coating a workpiece with paint comprising: a spray gun having a spraying nozzle arranged to project a powder paint in the form of air-dispersed particles towards a workpiece; a delivery duct communicating with the spraying nozzle of the gun; air feed means for supplying air to the delivery duct; paint feed means for supplying paint to the spraying nozzle; a ionization circuit having one pole connected to at least one electrode adapted to electrostatically charge the paint particles and a second pole electrically connected to the workpiece.
In the progress of the present description reference will be particularly made to painting of workpieces by electrostatic coating of same with paints in the form of dry powders, that is in the absence of liquid sol vents. However, the innovatory concept envisaged by the present invention can be utilized to advantage also for electrostatic painting with the use of liquid paints to be atomized.
2. Prior Art
It is known that apparatus employed for electrostatic powder-painting are generally comprised of a container inside which the powder paint is held suspended in air by means of a fluidized bed which is created by blowing air through a filtering element laid down at the base of the container itself. Connected to the container's top portion is a Venturi-type admission valve which is operatively interposed between a feed duct, in turn connected to a compressed air feed source, and a delivery duct in turn connected to a paint spray gun. The forced passage of air through the admission valve, regulated to a predetermined flow rate, causes the powder mixed with the air held in the container to be drawn in, according to an adjustable flow rate.
The mixture consisting of air and suspended powder thus admitted to the delivery duct reaches the gun and is sprayed out of the gun itself through an appropriate spraying nozzle. Usually, disposed close to the spraying nozzle is one or more electrodes connected to the negative pole of an electric feeding circuit, in order to create an electric ionization field in close proximity to the spraying nozzle.
Under this situation, the paint particles that, together with the air stream, come out of the nozzle are electrostatically charged as they pass through a ionization field and will consequently adhere to the workpiece which is normally polarized with a sign opposite that of the particles.
It is useful to note that during this step the adhesion of the paint particles is exclusively due to electrostatic effects so that the paint layer coated on the workpiece is very delicate and can be removed or damaged very easily. The stabilization of the molecular bonds between the paint particles and the workpiece will take place only at a subsequent baking step.
The foregoing being stated, it is clear that presently electrostatic paintings carried out by the use of apparatus of the described type involve many limits and drawbacks essentially resulting from the fact that an important amount of the sprayed powder, in many cases exceeding 50%, is dispersed in the surrounding atmosphere instead of being coated on the workpiece.
Therefore the work environment where painting is executed is enclosed in appropriate spray booths with which suitable suction and filtering systems are associated for recovery of the important amounts of dispersed powder paint.
The installation of these systems, which must be capable of filtering important amounts of air in a time unit, has an important weight on the overall cost of the painting plant.
In addition, the necessity of recovering and reusing powders involves important problems each time the paint being used must be replaced by another paint of different type and/or color. In these cases, in fact, the whole painting plant needs to be stopped for several hours which are necessary to carry out the filter replacement and cleaning of all surfaces and ducts concerned with the paint passage, in order to prevent traces of the previously used paint from contaminating the new type of paint to be used.
It is well apparent that this problem represents a severe limitation to the flexibility of use of the painting plant; in fact in order to partly obviate this drawback paint replacements are usually carried out at given periods and after one type of paint has been used for several working days.
On the other hand, contrary to that which could appear at first sight, the paint coating on a workpiece cannot be improved by merely increasing the values of the electrode supply current for the purpose of improving the electrostatic-charge effects of the particles. In fact, when these current values exceed given limits, electric discharges are created between the electrodes and the workpiece and they can irreparably impair the final result.
Neither can be made attempts to increase the outflow speed of the air and particles from the spraying nozzle in order to make the particles reach the workpiece before their being dispersed in the surrounding atmosphere. In fact a too strong air stream would cause the removal of the particles coated on the workpiece, and would make the individual particles remain in the electric induction field for a shorter time.
In view of the foregoing, all attempts hitherto carried out in the art for the purpose of improving paint coating have been substantially addressed to the study of appropriate geometrical shapes and structures of the spraying nozzles and, above all, to the qualitative improvements of the electrodes and feeding circuits connected thereto. By the use of very sophisticated technologies some improvements have been achieved which, however, appear relatively small when compared to the additional costs that such technologies involve.
For example, in accordance with the most advanced and expensive construction solutions, the ionization field could be produced directly within the container inside which paint is held in suspension. The large available room makes it possible to employ a much greater number of electrodes than on the spraying nozzle of the gun. In addition, the paint particles remain for a longer time in the ionization field.
However the paint particles tend to lose their electrostatic charge during their travel from the container to the gun, along the delivery duct. Consequently, the yield increase is much lower than the expected one. In fact, when substantially flat metal surfaces are to be painted, the amount of paint dispersed in the work environment almost never lowers below 25-30%.
It is also to be pointed out that, under given situations such as in the presence of trihedron angles where undesired phenomena of magnetic interference are created, a correct paint coating is still more difficult, and sometimes even impossible. This fact gives rise to important problems, above all with reference to the modern painting plants of the automated type in which in many cases manual finishing interventions are required for executing the paint coating in those areas that can be hardly reached by paint.
Furthermore, the problems that are presently connected with the electrostatic painting make it practically impossible to use this process for paint coating manufactured articles made of a material of low conductivity, such as glass, as well as for coating additional paint layers on articles painted during a previous working step.
In accordance with the present invention, it has been found possible to solve a preponderant part of the problems of the known art if at least one additive gaseous fluid is admixed with the air conveyed to the spray gun nozzle, which additive fluid will have a greater electric conductivity than the air.